The present invention relates to a combustion control apparatus and method for an engine for switching the combustion mode between a stratified-charge combustion mode and a homogeneous-charge combustion mode.
Engines that selectively perform homogeneous-charge combustion, which forms a homogeneous air-fuel mixture layer in the combustion chamber, and stratified-charge combustion, which forms a rich air-fuel mixture layer in the vicinity of the ignition plug, are known.
Such engines reduce the pumping loss and cooling loss to improve fuel efficiency, as compared with an engine that performs only homogeneous-charge combustion. In a heavy-load operation, which increases the amount of fuel injection, the combustion mode is switched to homogeneous-charge combustion from stratified-charge combustion to provide the desired engine power.
For example, in the engine disclosed in Japanese Unexamined Patent Publication (KOKAI) No. Hei 10-176570, when abnormal combustion due to self-ignition of the air-fuel mixture, or knocking, occurs, the ignition timing is retarded to suppress the maximum combustion pressure. This suppresses knocking. This control procedure is called xe2x80x9cknocking control.xe2x80x9d
Knocking control changes the retardation of the ignition timing in accordance with the frequency of knocking occurrences. When knocking occurs frequently, the retardation of the ignition timing is increased.
In stratified-charge combustion mode, it is necessary to ignite the fuel when an air-fuel mixture layer of a predetermined concentration is formed in the vicinity of the ignition plug. The stratified-charge combustion mode has a narrower range within which the ignition timing can be changed, as compared with homogeneous-charge combustion. If the ignition timing is retarded in the stratified-charge combustion mode to suppress knocking, as in the homogeneous-charge combustion mode, the ignition timing departs from the allowable range. This degrades the combustion and sometimes results in misfires.
If the operation range of an engine that performs stratified-charge combustion is limited to a range where knocking does not occur, combustion deterioration can be avoided; however, this reduces the operation area of the engine in which stratified-charge combustion is performed, which lowers the fuel efficiency.
Accordingly, it is an object of the present invention to provide a combustion control apparatus and method for an engine which can adequately execute knocking control without adversely affecting the execution of stratified-charge combustion.
To achieve the above objective, the present invention provides an apparatus for controlling combustion in an engine that produces power by burning a mixture of air and fuel in a combustion chamber. The engine performs a mode of combustion selected from a plurality of combustion modes. The combustion modes are stratified-charge combustion, in which the mixture is distributed in a concentrated manner in the combustion chamber, and homogeneous-charge combustion, in which the mixture is distributed in a homogeneous manner in the combustion chamber. The apparatus comprises a controller for switching the combustion mode in accordance with current running conditions of the engine. The controller limits a running area in which stratified-charge combustion is selected in accordance with the occurrence of engine knocking.
To achieve the above objective, the present invention also provides a method for controlling combustion in an engine that produces power by burning a mixture of air and fuel in a combustion chamber. The engine performs a combustion mode selected from a plurality of combustion modes. The combustion modes are stratified-charge combustion, in which the mixture is distributed in a concentrated manner in the combustion chamber, and homogeneous-charge combustion, in which the mixture is distributed in a homogeneous manner in the combustion chamber. The method comprises switching the combustion mode in accordance with current running conditions of the engine, and limiting a running area of stratified-charge combustion in accordance with the frequency of engine knocking occurrences.
Other aspects and advantages of the invention will become apparent from the following description, taken in conjunction with the accompanying drawings, illustrating by way of example the principles of the invention.